In-place reserves of heavy oil in the United States have been estimated about one hundred fifty billion barrels. Of this large in-place deposit total, however, only about five billion barrels may be considered economically produceable at current oil prices. One major impediment to production of oil from such deposits is the high viscosity of the oil. The high viscosity reduces the rate of flow through the deposit, particularly in the vicinity of the well bore, and consequently increases the capital costs per barrel so that overall costs per barrel become excessive.
Various techniques have been tried to stimulate flow from wells in heavy oil deposits. One technique utilizes steam to heat the oil around the well; this method has been utilized mostly in California. However, steam has drawbacks in that it is not applicable to thin reservoirs, is not suitable for many deposits which have a high clay content, is not readily applicable to off-shore deposits, and cannot be used where there is no adequate water supply.
There have also been a number of proposals for the use of electromagnetic energy, usually at conventional power frequencies (50/60 Hz) but sometimes in the radio frequency range, for heating oil deposits in the vicinity of a well bore. In field tests, it has been demonstrated that electromagnetic energy can thus be used for local heating of the oil, reducing its viscosity and increasing the flow rate. A viscosity reduction for oil in the immediate vicinity of the well bore changes the pressure distribution in the deposit to an extent such that flow rates may be enhanced as much as three to six times.
Perhaps the most direct and least costly method of implementation of electromagnetic heating of deposits in the vicinity of a well bore utilizes existing oil well equipment and takes advantage of conventional oil field practices. Thus, conventional steel well casing or production tubing may be employed as a part of a conductor system which delivers power to a main heating electrode located downhole in the well, at the level of the oil deposit. However, the high magnetic permeability of a steel casing or tubing, with associated eddy current and hysteresis losses, often creates excessive power losses in the transmission of electrical energy down the well to the main electrode. Such power losses are significant even at the conventional 50/60 Hz supply frequencies that are used almost universally. These losses may be mitigated by reducing the A.C. power frequency, as transmitted to the downhole heating electrode, but this expedient creates some substantial technical problems as regards the electrical power source, particularly if the system must be energized from an ordinary 50/60 Hz power line.
Various power sources could be used for low frequency electromagnetic heating of the producing deposits around oil wells or other mineral fluid wells; for example, a conventional motor generator set could be employed. To generate really low frequencies by means of a motor generator set, as in a range below thirty-five Hz, however requires a very large generator that incorporates a great deal of iron. As a consequence, such a motor generator set is unduly costly and may also be quite difficult to maintain.
Another possible heating source is an amplifier of the conventional audio frequency type. In a source of this kind the usual 50/60 Hz power line voltage is first rectified and is then used to energize a conventional but high power audio frequency amplifier operating at the desired low frequency. But a power source of this kind is not really desirable because such amplifiers are relatively wasteful, usually operating at efficiencies of only about sixty to eighty percent.
Even if such conventional low frequency power sources were otherwise acceptable, their routine application to heating the producing zone around the wellbore of a heavy-oil well may pose costly difficulties. The nature of the formations and the flow rates of the produced fluids change. Such changes may lead either to formation damage or to damage or destruction of the downhole equipment. A small and controllable D.C. component, in combination with the larger low frequency A.C. heating current, may also be needed for corrosion protection. This might be accomplished by placing a conventionally designed controllable source of D.C. power in series with one of the aforementioned conventionally designed sources of low frequency A.C. power, but the cost of such a D.C. supply, which would have to be capable of withstanding hundreds of amperes of low frequency A.C. current, is excessive and renders such conventional equipment impractical. Furthermore, such combinations of conventionally designed equipment are not likely to meet the requirements of electric power utilities for minimizing power rates while simultaneously being responsive to changes occurring in the formations being heated or to variations of the specific heat or flow rates of the produced fluids.
There is another type of oil well heating system in which the heat is applied to the flow of oil within the well itself, rather than to a localized portion of the deposit around the well. Such a heating system, usually applied to paraffin prone wells but also applicable to other installations, is described in Bridges et al U.S. Pat. No. 4,790,375 issued Dec. 13, 1988. In a system of this kind the heating element or elements constitute the well casing, the production tubing, or both; the high hysteresis and eddy current losses in steel tubing may make its use advantageous. In such systems it may be desirable to supply heating power to the system at frequencies substantially above the normal power range of 50/60 Hz; otherwise, the problems may be similar to the low frequency systems previously mentioned.